What is the recommended imaging modality for measuring retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL) thickness in myopic patients?

Imaging Modalities for Measuring RNFL and GCL Thickness in Myopic Patients

Spectral Domain Optical Coherence Tomography (SD-OCT) is the recommended imaging modality for measuring retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL) thickness in myopic patients, but interpretation must account for the inherently thinner RNFL and GCL in high myopia to avoid false glaucoma diagnoses. 1, 2, 3

Main Imaging Modalities

Spectral Domain OCT (SD-OCT)

SD-OCT has emerged as the primary imaging technique for measuring RNFL and GCL thickness due to its:

• High resolution: Provides detailed cross-sectional images of retinal layers
• Reproducibility: Offers consistent measurements for longitudinal monitoring
• Quantitative analysis: Generates numerical data for objective assessment

Specific SD-OCT systems used in research include:

• RTVue-100 (with NHM4 and MM7 scan protocols) 4
• Cirrus HD-OCT 5

Accuracy and Advantages

1. Strong correlation between measurements:

   • GCC thickness shows strong correlation with RNFL thickness (correlation coefficient=0.763, p<0.001) 4
   • Both measurements effectively detect structural changes in myopic patients

2. Complementary measurements:

   • Macular GCC thickness measurements may be a good alternative or complementary to RNFL thickness assessment in myopic patients 4
   • In some cases, GCC measurements may have higher sensitivity than RNFL for detecting glaucomatous changes in myopic eyes 4

3. Quantifiable relationships with myopia:

   • Average RNFL thickness correlates with:
     • Spherical equivalent (3.667 μm/diopter) 3
     • Axial length (-5.3805 μm/mm) 3
     • Optic disc area (9.4617 μm/mm²) 3
   • Average GCIPL thickness correlates with:
     • Spherical equivalent (1.6807 μm/diopter) 3
     • Axial length (-2.626 μm/mm) 3

Limitations in Myopic Patients

1. Thickness variations with myopia severity:

   • High myopia is associated with significantly thinner RNFL and GCC compared to low and moderate myopia 2, 3, 5
   • Quadrantic RNFL thickness profiles are thinner in high myopia except for the temporal quadrant 3
   • Average, superior, and inferior GCC values are significantly lower in high myopia 2

2. Impact of axial length:

   • Negative correlation (-0.712, p<0.001) between RNFL thickness and axial length 1
   • Average RNFL thickness decreases by approximately 1.44-5.38 μm per mm increase in axial length 3, 5
   • Average GCIPL thickness decreases by approximately 1.65-2.63 μm per mm increase in axial length 3, 5

3. Optic disc area influence:

   • Larger optic discs are associated with increased RNFL thickness but not GCIPL thickness 3, 5
   • Average RNFL thickness increases by approximately 5.35-9.46 μm per mm² increase in optic disc area 3, 5

Clinical Implications and Pitfalls

1. Risk of misdiagnosis:

   • Highly myopic patients have inherently thinner RNFL (mean 80.0 μm vs. 108.8 μm in non-myopic controls) 1
   • Without accounting for myopia, there's significant risk of false glaucoma diagnosis 1, 5

2. Recommended approach:

   • Use both macular GCC/GCIPL and peripapillary RNFL measurements for comprehensive assessment 4
   • Consider using GCC/GCIPL measurements as they may be less affected by optic disc size variations 3, 5
   • Interpret OCT results in the context of refractive status and optic disc size 5

3. Diagnostic performance:

   • In high myopia, GCC measurements may have slightly better diagnostic ability for glaucoma (AUROC 0.889) compared to RNFL (AUROC 0.825), though not statistically significant 4
   • Superior GCC color code has shown significantly higher sensitivity than superior RNFL for detecting glaucomatous changes in myopic patients 4

When evaluating myopic patients, clinicians should be aware that normative databases in OCT devices may not adequately account for the structural variations in high myopia, necessitating careful interpretation of results and consideration of both RNFL and GCC/GCIPL measurements for accurate assessment.